R. Dal Maschio

Influence of sol-gel coatings on crack initiation by vickers indentation in soda-lime glass

Abstract In this work an indentation technique was used to study the effect of the sol-gel coatings of soda-lime glasses on crack formation. The tendency of samples to develop radial cracks has been evaluated by measuring their number in correspondence with the Vickers impressions as a function of the applied load. Silica coatings of different thickness were obtained from alcoholic solutions of TEOS by the dipping technique on glass sheets with different degrees of surface hydration. All the coated samples show a lower crackability compared with the uncoated ones. It was found that this parameter is influenced by layer thickness, heat treatment temperature and by the degree of galss surface hydration.

Mössbauer and adhesion study of H+ implanted iron films on insulators

Abstract Depth-selective surface Mossbauer analyses and scratch tests were performed for iron films on various substrates before and after proton bombardment. The results show that the adhesion behaviour is related to contaminants and mechanical stresses at the interface.

Mechanical properties of ion implanted glasses

Abstract It is well known that heavy ion implantation causes modifications of the surface mechanical characteristics of glasses. At first a compaction is induced with increasing ion fluence, then a relief is observed after a maximum tension stress. In this paper we report the results of a study on the mechanical resistance of Ar implanted soda-lime glasses. Vickers microindentation tests have been performed by measuring both the percentage of cracks, which develop after application of a loaded square pyramidal indenter on the glass, with respect to the number of tests and the length of cracks. In order to clarify the mechanisms responsible for the ion induced mechanical modifications, the influence of the implantation parameters and the thermal stability of the modified surface layers have been investigated.

Fabrication and characterization of polymer-derived Si2N2O-ZrO2 nanocomposite ceramics

Amorphous Si-Zr-N-O powders, obtained by nitridation in an NH3 flow of zirconium modified polycarbosilane, have been sintered to full density by hot pressing at 1500‡C. The resulting ceramic shows an extremely fine-grained microstructure composed of Si2N2O and ZrO2 crystallites 20–30 nm in diameter. Thermal stability measured in air appears excellent up to 1300‡C for 48 h. Mechanical characterization pointed out good values of flexural strength (330 MPa), fracture toughness (4.1 MPam0.5) and Weibull modulus.

Proton beam enhanced adhesion of iron films

Depth-selective surface Mössbauer and adhesion measurements were pertormed on as evaporated and proton bombarded57Fe and56Fe/57Fe films supported by glass and fused silica. The results contribute to the understanding of the mechanism responsible for adhesion enhancement of metallic films induced by ion bombardment in the electronic stopping region.

Radial crack initiation in glass by Vickers indentation

Abstract A statistical model for radial-crack initiation using Vickers indentations is suggested. By means of such a model it is possible to calculate the threshold load, defined as the load at which there is a 50% fracture probability, from experimental evaluations. At full loading, shear stress is the driving force to crack-nuclei formation; on these grounds a proportionality between the load at which there is a crack-initiation probability and the value of the sliding size under that load on the whole indentation has been suggested. Lastly, using the Hagan formulation and threshold load experimental values, it has been possible to calculate K I c values in substantial accordance with other methods.

Buried Optical Guide Formation

A systematic study of the mechanisms which govern the in-depth migration of Ag ions in glasses, in presence of an external field, has been performed, with the aim to create buried waveguides. Ag films, from 60 to 240nm thick, were evaporated on the surface of glasses of different composition. Electric fields ranging from 300 to 1000 V/cm, were applied across the glasses for different times and temperatures. The Ag distribution and glass composition modifications were analyzed using nuclear techniques, as RBS and NRA. Stresses induced by the process were investigated using mechanical microindentation techniques. A comparison between theoretical and experimental profiles has been performed and the importance of the different parameters (electric field, glass temperature, Ag surface layer thickness, process duration) was enlighted.

Crystallization of silica in SiO2-Fe2O3 glasses obtained from gels

Abstract The effect of iron oxides on silica crystallization was investigated on SiO2 -Fe2O3 glasses prepared by the sol-gel method. Heat treatment of gels under oxygen and hydrogen fluxes gave oxidized and reduced iron in glass, while different redox ratios were achieved under nitrogen in different compositions. Crystallization of these samples at different temperatures was studied by X-ray diffraction. quartz was the only ordered phase to be detected and the influence of redox ratio on its formation was verified.

Crack nucleation in ultrathin blown silicate glasses

Abstract Ultrathin blown glasses, owing to the very high cooling rate, have an open structure, characterized by lower hardness and higher deformability, and no tempering residual stresses. In these glasses, due to their high densificability, the threshold load for crack nucleation is higher than in air-cooled plates, and it is also above the values estimated by present theoretical formulations.

Unusual thermal expansion behaviour of PbO-rich PbO-B2O3 glasses

An unusual discontinuity in the thermal expansion behaviour of a 36 PbO-64 B2O3 wt % phase-separated glass was examined. Expansion tests combined with other experimental techniques established a correlation with a structural rearrangement in the cracked particles of the dispersed phase.